Indent the outer argument if it fits in one line, rather than the inner one

The title may sound cryptic but this is what I mean

```
$ echo 'a_very_indented_function(another_very_long_function(key0=val0, key1=val1, key2=val2))' | yapf
a_very_indented_function(another_very_long_function(key0=val0,
key1=val1,
key2=val2))
```

I'd argue

```
a_very_indented_function(
another_very_long_function(key0=val0, key1=val1, key2=val2))
```

is clearly better.